In the field of the manufacture of tires and in particular of the formulation of rubber compositions in contact with the ground, known as treads, the aim is generally to find the best possible compromise between conflicting performances, such as the behaviour of the vehicle, the rolling resistance, the dry and wet grip, and the wear.
In particular, it would be desirable to reduce the modulus of the materials forming the treads in order to promote grip and the indentation of the tread in the ground but, at the same time, treads of very high stiffness would be preferred for wear and road behaviour performances.
Any modification to the formulation of the treads has consequences for the compromise between these performances.
It is known that it is possible to improve the grip of tires under forceful braking conditions without modifying the hysteresis under normal rolling conditions and thus without modifying the rolling resistance of the tires concerned.
In particular, it is known that the above compromise can be improved by producing rubber compositions which result in treads having a modulus of elasticity which greatly decreases at the surface as a result of a rise in temperature. This is because such a decrease in the modulus, for example during a braking action which is always accompanied by a local increase in the temperature, makes it possible to increase the degree of effective contact of the tread with the ground, in particular rough ground, and thereby to enhance the performances related to the contact, in particular ground grip, and thus the braking capability.
This result can be obtained by introducing, into a rubber composition for the manufacture of tires based on one or more diene elastomers, on one or more reinforcing fillers and on a vulcanization system, particles of one or more thermoplastic materials.
There is known, from the document WO 2004/039872, a process for the preparation of a composition based on diene elastomer and on thermoplastic polymer chosen from amorphous polymers with a glass transition temperature of greater than 80° C. and crystalline polymers with a melting point of greater than 190° C. The process comprises a stage of premixing the thermoplastic polymer with a portion of the diene elastomer, in order to obtain a masterbatch, this premixing being carried out at a temperature equal to or greater than the glass transition temperature or equal to or greater than the melting point reduced by 20° C., and then a stage of mixing the masterbatch with the remaining portion of the diene elastomer.
However, the use of such compositions can result in a large decrease in the ultimate strength, which may be damaging in terms of wear.